In many cells, release of Ca from intracellular stores is followed by Ca influx from the extracellular space ("capacitative Ca entry") through store-operated Ca channels (SOCCs). Ca influx through SOCCs is important for many cellular activities including activation of T lymphocytes by antigen, secretion of insulin from pancreatic acinar cells in response to secretagogues, and stimulation of mast cell degranulation. Despite the importance of SOCCs, very little is known about their molecular structure or control mechanisms, but an ion channel (TRP) cloned from Drosophila photoreceptors may provide insights into SOCC structure and function. Recently, highly conserved homologs of TRP have been identified in several species and it appears that TRP and these homologs are SOCCs. The purpose of this proposal is to test the hypothesis that TRP and its homologs are SOCCs and to investigate the structure, function, and regulation of TRPs using biophysical and molecular genetic approaches. Specific aims will be to study the cellular distribution of TRP homologs in human tissues and cell lines, to determine the topology of TRP homologs in the membrane, to characterize the properties of TRP ion channel homologs expressed in Xenopus oocytes and mammalian cell lines, to identify accessory subunits of the channel, and to develop C. elegans as a genetic system for studying TRP ion channels. These studies are expected to provide valuable knowledge about the mechanisms by which Ca is regulated in cells. Because intracellular Ca is a key element in signal transduction cascades in virtually every cell type, we anticipate that a better understanding of this fundamental cell biological process will provide new insights into a variety of human diseases.